MANCHESTER, England, Sept 15 (Reuters) - Manchester United manager Alex Ferguson said on Friday his Premier League leaders would not underestimate Arsenal when they face their old foes at Old Trafford on Sunday.

United have enjoyed their best start to a Premier League season with four wins from four, while a mis-firing Arsenal lie 17th after taking just two points from their three games.

"I don't think we can under-estimate Arsenal," Ferguson told reporters. "We will need to play to our best capabilities, but I am encouraged with the atmosphere around the place, and the team spirit."

Ferguson believes that Arsenal, who finished fourth in the league last season, are feeling the effects of losing some of their key players.

"Maybe there is a transitional period at Arsenal at the moment because they have lost big figures like (Patrick) Vieira, (Dennis) Bergkamp, (Robert) Pires and Ashley Cole.

"Just like us, we lost big players and you have to be patient for the next stage of development."

United's clashes with Arsenal have been tempestuous affairs in past years.

Although some of the sting has been removed by the departures of midfield adversaries Roy Keane and Vieira from their respective clubs, Ferguson still expects a demanding time on Sunday.